a person who is narrow-minded or has limited knowledge or insight to anything outside his or her immediate life.

basically someone who lacks education or higher thought.

I tried to start a conversation about Hapsburg Europe with that hot looking babe at the party who is supposedly a history major.
turns out she's just another mental midget, since all she talked about was her ex boyfriend Diego and her Mary Kay cosmetics collection!

A person with a fragile mindset and lacking the ability to deal with tough situations. Feeble.

Whenever Mitch is given constructive criticism, he turns into a total mental midget and starts freaking out thinking he is going to get fired and becomes all pissy and whiny. He can't mentally deal with pressure and folds like a cheap lawn chair.